Search Images Maps Play YouTube News Gmail Drive More »
Sign in
Screen reader users: click this link for accessible mode. Accessible mode has the same essential features but works better with your reader.

Patents

  1. Advanced Patent Search
Publication numberUS4101195 A
Publication typeGrant
Application numberUS 05/820,498
Publication dateJul 18, 1978
Filing dateJul 29, 1977
Priority dateJul 29, 1977
Publication number05820498, 820498, US 4101195 A, US 4101195A, US-A-4101195, US4101195 A, US4101195A
InventorsA. Administrator of the National Aeronautics and Space Administration with respect to an invention of Frosch Robert, Dietrich G. Korsch
Original AssigneeNasa, Korsch Dietrich G
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Anastigmatic three-mirror telescope
US 4101195 A
Abstract
A three-mirror telescope having an ellipsoidal primary mirror, a hyperbolic secondary mirror, and an ellipsoidal tertiary mirror, with the three producing an image in a conveniently located finite plane for viewing.
Images(2)
Previous page
Next page
Claims(4)
What is claimed is:
1. A three-mirror telescope comprising:
a primary mirror of ellipsoidal configuration, and having an opening central about a central axis thereof;
a secondary mirror of hyperboloidal configuration facing said primary mirror, said secondary mirror being of a smaller diameter than the primary mirror and positioned symmetrically with the axis of the primary mirror, whereby light from a scene being viewed passes around and by the secondary mirror, and then is reflected through said opening in said primary mirror; and
imaging means, including a tertiary mirror of ellipsoidal configuration positioned to receive light from said secondary mirror, for reflecting an image of the scene being viewed to a plane for viewing.
2. A telescope as set forth in claim 1 wherein said imaging means comprises a plane mirror positioned in an optical path including said tertiary mirror and is oriented at an angle of 45 with respect to said axis.
3. A telescope as set forth in claim 2 wherein said tertiary mirror is positioned symmetrical with the optical axis of the telescope and with said opening in said primary mirror, and said plane mirror is positioned and sized to enable light to pass around it from said secondary mirror to said tertiary mirror and to receive reflected light from said tertiary mirror and project an image in a plane which is parallel to but spaced from said axis.
4. A telescope as set forth in claim 2 wherein said plane mirror has a central opening, said tertiary mirror is positioned symmetrically about a line perpendicular to said axis and receives reflected light from said plane mirror and reflects this light back through said opening in said plane mirror, whereby a final image is created in a plane which is parallel with but displaced from said axis.
Description
ORIGIN OF THE INVENTION

The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2457).

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to telescopes, and particularly to an improved three-mirror telescope.

2. General Description of the Prior Art

The performance of a conventional high quality telescope when used on the earth for celestial viewing is principally limited to the earth's atmosphere rather than by the construction of the telescope. Atmospheric effects not only limit the resolution of a telescope to approximately one arc second, but also absorb large portions of the electromagnetic spectral range. While a decade or so ago little could be done about this limitation, today, with the help of earth satellites and other space vehicles, it is possible to place a telescope above the atmosphere and perform extraterrestial observations without interference from it. As a result, there has arisen a need for a telescope which can more fully take advantage of this new environment. Sought is a telescope which has a higher resolution over a wider field of view (equal to or greater than 1) and one having a greater spectral range, ideally extending from the far ultraviolet to the far infrared. It is immediately seen that to obtain the latter feature, the telescope must be an all-reflective type in order to avoid lens transmission losses, particularly at extremes of the spectral range. The first choice from existing types of reflecting telescopes would appear to be the Richey-Chretien telescope, which is an improved version of the classical Cassegrain telescope. The Ritchey-Chretien telescope is a two-mirror telescope and can, unfortunately, provide a high resolution field of only a few arc min, and it outputs a curved image field. To widen and flatten the field of this telescope, it is normally used in conjunction with refractive correctors, but refractive correctors are essentially operative only in the visual portion of the spectrum, and thus the desired wider spectral range cannot be obtained with this telescope.

As perhaps a most logical step toward improvement, several three-mirror telescopes have been proposed as possible solutions, and these have been described in the following publications:

1. Paul, M. Revue D'Optique, 14, No. 5, 1935, p.13.

2. Baker, J. E., IEEE Trans. Aer. E1. Sys., Vol. AES-5, No. 2, 1969.

3. Lagrula, Jr., Cahiers de Phys., 1942, pp. 8-43.

4. Korsch, D., Appl. Opt., Vol. 11, No. 12, 1972, p. 2986.

5. Shack, R. V. and Meinel, A.B., J.O.S.A., 56, 1966, p. 545.

6. Rumsey, N. J., Proceedings, Opt. Instru. and Techniques, 1969, Oriel Press, p. 514.

7. Buchroeder, R. A. and Leonard, A. S., Appl. Opt., Vol. 11, No. 7, 1972, p. 1649.

8. Buchroeder, R. A., Technical Report No. 68, Opt. Sc. Center, University of Arizona, 1971.

9. Korsch, D., J.O.S.A., Vol. 63, 1973, p. 667.

10. Korsch, D., Appl. Opt., Vol. 13, 1974, p. 1767.

Despite these efforts, an examination of the three-mirror systems proposed by them indicates that none of them provide practical and sufficient solutions to the problems involved, each of the systems having one or more of the following short-comings:

1. Inaccessability of the image plane.

2. A large central obscuration.

3. Practically invariable fast focal ratios.

4. A largely asymmetric configuration.

In addition to the three-mirror telescopes disclosed by the foregoing references, two additional three-mirror telescopic designs have more recently appeared, one by the Itek Corporation, described in "Requirements and Concept Design for Large Earth Survey Telescope for SEOD", Final Report, NASA CR-144796 (1975), and one by the applicant, described in Optical Engineering, Vol. 14, No. 6, (1975), p. 533. While these mirror designs offer some advantage over previous ones, they still suffer the disadvantage that, because of their geometric configurations, only less than half of the well-corrected field can be used.

SUMMARY OF THE INVENTION

In accordance with the present invention, the aforementioned difficulties are quite substantially overcome by a three-mirror telescope wherein a primary-secondary configuration similar to the Cassegrain design forms a real image closely behind the primary mirror. This image is then re-imaged by a tertiary mirror at approximately unit magnification in a finite plane for viewing, this plane being parallel to but displaced from the axis of the primary and secondary mirrors. To achieve this, a plane mirror, which functions as a fold mirror, is positioned at an angle of 45 with respect to the image formed by the primary-secondary mirrors and either directs light from the secondary mirror to the tertiary mirror or reflects light from the tertiary mirror to a final image plane.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of the cross section of a three-mirror telescope as contemplated by this invention.

FIG. 2 is a schematic illustration of a modification of the telescope shown in FIG. 1.

FIG. 3 is a schematic illustration of the field distribution as would be observed at the final image plane of a telescope constructed in accordance with this invention.

FIG. 4 is a graph illustrating a comparison of the performance of a telescope constructed in accordance with this invention and a conventional Ritchey-Chretien telescope.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1, primary mirror 10, having a central opening 11, is positioned at end 12 of tube or tubular housing 14 and is an ellipsoid. Secondary mirror 16, which is supported in tube 14 by conventional spider support members 18, is a hyperbolic mirror. Tertiary mirror 20, supported by auxiliary support housing 22 (by means not shown), is an ellipsoid. Plane mirror 24, which is supported on spider members 26 at an angle of 45 with respect to the axis 28 of telescope 30, is made small to permit light to pass around it from secondary mirror 16 to tertiary mirror 20 and yet effectively receive light from tertiary mirror 20 and reflect it upward to image plane 32 of camera 34. Instead of a camera, an eyepiece could be positioned, for viewing through the telescope, at image plane 32.

Thus, in operation, rays 36 enter end 38 of tube 14 and pass around secondary mirror 16 and impinge on primary mirror 10 which reflects the rays onto secondary mirror 16. Rays 36 are then reflected by secondary mirror 16 onto tertiary mirror 20. Tertiary mirror 20 reflects the rays onto plane mirror 24, and this mirror reflects the rays upward to focal plane 32 where they are observed and recorded by camera 34, or otherwise viewed. The rays in FIG. 1 show the outside limits of the envelope of light throughout the telescope.

In the alternate configuration shown in FIG. 2, a larger plane or fold mirror 40 directly receives rays 36 from secondary mirror 16 and reflects them to tertiary mirror 20. Rays 36 from tertiary mirror 20 are then reflected through an opening 42 in plane mirror 40 to final image plane 32. This configuration minimizes obscuration by avoiding the use of a spider 26, as shown in FIG. 1, that holds a small fold mirror 24, and at the same time significantly improves the baffling of the system. In fact, an additional advantage of this three-mirror configuration over the Cassegrain two-mirror configuration is its natural baffling property. In contrast, in the Cassegrain telescope, in order to protect the secondary image from stray light, fairly complex and elaborate baffling systems are required. Thus, in the present invention, the final image plane is inherently protected from stray light by the folded-out image plane and the location of exit pupil 42 being behind the tertiary mirror, which together form a bottleneck in the optical train. More particularly, with respect to the embodiment of the invention shown in FIG. 1, the only stray light that can reach the final image plane is that scattered off spider members 26 holding small fold mirror 24. However, the spider members are so far within the optical system that they will not be illuminated by the moon, earth or sun.

An even more efficient baffling effect is achieved with the configuration shown in FIG. 2. With this configuration, no stray light can reach an image plane after only a single scattering event. Even the light that is scattered off the edges around perforation 42 of fold mirror 40, and then reflected by the tertiary through exit pupil 42, will be intercepted by the central vignetted portion 44 rather than by the useful field. A further advantage is the accessability of the image of the spider holding the secondary mirror as formed by the tertiary mirror. It is located immediately behind exit pupil 42 and can, at least in the configuration of the invention shown in FIG. 2, easily be masked off, e.g., by a screen shape like the image of the spider.

The performance obtainable from telescopes constructed in accordance with this invention is demonstrated in FIG. 4 where such performance is compared to the performance of a Ritchey-Chretien telescope. The geometric spot size, i.e., the diameter of the smallest circle surrounding all rays traced through this system, is plotted as a function of field angle. As shown, the superior performance of such a three-mirror telescope is reflected in the significantly smaller spot size and in the fact that the field is flat, while the Ritchey-Chretien telescope has a curved field. The superiority of the three-mirror telescope of this invention over two-mirror telescopes is further demonstrated by the fact that while the latter can be corrected for maximally two aberrations, usually spherical aberration and coma, the former can be corrected for four aberrations, spherical aberration, coma, astigmatism, and field curvature. The mathematical conditions for correcting spherical aberrations, coma, and astigmatism can be written according to equations previously published by the applicant in J.O.S.A., Vol. 63, 1973, p. 667, and in Applied Optics, Vol. 13, 1974, p. 1967; and as an example, a list of final telescope parameters for a telescope constructed in accordance with the present invention would be as follows:

______________________________________Clear aperture     150 cmPrimary F-No.      2.2System F-No.       12System focal length              1,800 cmSecondary diameter 35 cmTertiary diameter  80 cmExit pupil diameter              5.2 cmSecondary image diameter              48.3 cm (1.5)Final image diameter              47.1 cm (1.5)Primary radius     660.0000 cmSecondary radius   -126.9495 cmTertiary radius    154.2855 cmPrimary deformation              -0.9702785 (ellipsoid)Secondary deformation              -1.7448287 (hyperboloid)Tertiary deformation              -0.5596906Secondary magnification              -5.6Tertiary magnification              0.9740Distance:Primary-secondary  277.8600 cmSecondary-tertiary 448.3266 cmTertiary-exit pupil              92.0000 cmExit pupil-image plane              62.2817 cm______________________________________

In order to dispel any apprehension concerning the possibility of a drastic increase in complexity due to the addition of the tertiary mirror, the following table indicates the effect of secondary and tertiary misalignment on the performance in terms of RMS wavefront errors (optical path difference) and in terms of induced aberrations (increase of spot size):

______________________________________INCREASE OF RMS-OPD INCREASE OF GEOMETRICPER UNIT MISALIGNMENT               SPOT DIAMETER(λ = 632.8 nm)               PER UNIT MISALIGNMENT______________________________________SECOND-ARYDespace  0.025 λ/μm                   0.032 μ rad/μmDecenter  0.0013 λ/μm                   0.0036 μrad/μmTilt   0.0008 λ/μrad                   0.0023 μrad/μradTER-TIARYDespace  0.0016 λ/μm                   0.0021 μrad/μmDecenter  0.016 λ/mm                   0.048 μrad/mmTilt   0.004 λ/mrad                   0.014 μrad/mrad______________________________________

From the foregoing table, it will be noted that the tertiary is 15 to 200 times less sensitive than the alignment of the secondary mirror.

In summary, it is believed clear that the present invention provides a most feasible and practical solution to the problem of the construction of a highly corrected all-reflective telescope. In addition to its superior performance as described, the annular field of its output is particularly adapted to accommodate a set of different instruments, with each being assigned a discrete portion of the field. This enables a space vehicle to be more versatile, it allows for the accommodation of back-up instruments which may be used in the case of a failure, and generally facilitates the collection of maximum data per space mission.

Patent Citations
Cited PatentFiling datePublication dateApplicantTitle
US3674334 *Jan 4, 1971Jul 4, 1972Perkin Elmer CorpCatoptric anastigmatic afocal optical system
Non-Patent Citations
Reference
1 *Gelles, App. Optics, vol. 12, No. 5, May, 1973, p. 935.
Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US4221459 *Aug 7, 1978Sep 9, 1980Fisher William EErect image telescope optical system
US4265510 *May 16, 1979May 5, 1981Hughes Aircraft CompanyThree mirror anastigmatic optical system
US4444474 *Jan 25, 1982Apr 24, 1984Pasko Edward HStationary eyepiece telescope
US4598981 *Feb 5, 1985Jul 8, 1986The United States Of America As Represented By The Administrator Of The National Aeronautics And Space AdministrationWide-angle flat field telescope
US4754381 *Jun 26, 1987Jun 28, 1988Downs James WEllipsoidal reflector concentration of energy system
US4834517 *Jan 13, 1987May 30, 1989Hughes Aircraft CompanyMethod and apparatus for receiving optical signals
US4883348 *Jun 10, 1988Nov 28, 1989Spivey Brett AWide field optical system
US4993818 *Oct 17, 1988Feb 19, 1991Hughes Aircraft CompanyContinuous zoom all-reflective optical system
US5107369 *Nov 28, 1990Apr 21, 1992Thermo Electron Technologies Corp.Wide field multi-mode telescope
US5132836 *Jul 15, 1991Jul 21, 1992Fundingsland John OReflecting telescope including a flat apertured mirror rotatable about perpendicular axes
US5136422 *Oct 31, 1990Aug 4, 1992Horiba, Ltd.Reflective optical system for a microscopic spectrometer
US5170284 *Aug 16, 1991Dec 8, 1992Hughes Aircraft CompanyWide field of view focal three-mirror anastigmat
US5239404 *Sep 4, 1992Aug 24, 1993Litton Systems, Inc.Large angle reflective scanning system and method
US5386316 *Feb 8, 1993Jan 31, 1995Hughes Aircraft CompanyOptical systems having multiple simultaneous functions
US5430577 *Dec 10, 1991Jul 4, 1995Karl F. AngstenbergerDouble reflector
US5555041 *Apr 3, 1995Sep 10, 1996Nikon CorporationProjection apparatus
US5631770 *May 26, 1994May 20, 1997Hughes Danbury Optical Systems, Inc.Reflective scanning telescopic system on single optical bench
US5661610 *Oct 17, 1994Aug 26, 1997Matra Marconi Space France S.A.Telescope for infrared or visible imaging
US5734496 *Nov 18, 1994Mar 31, 1998Her Majesty The Queen In Right Of New ZealandLens system
US5737137 *Apr 1, 1996Apr 7, 1998The Regents Of The University Of CaliforniaCritical illumination condenser for x-ray lithography
US5995280 *Feb 24, 1997Nov 30, 1999Her Majesty The Queen In Right Of New ZealandLens system
US6819483 *Aug 31, 2001Nov 16, 2004Lockheed Martin CorporationOptical device and method for correcting field-dependent phase errors in distributed aperture telescope systems
US6850361 *Jun 9, 2000Feb 1, 2005Mitsubishi Denki Kabushiki KaishaWide-angle catoptric system
US6886953Feb 25, 2003May 3, 2005Raytheon CompanyHigh-resolution, all-reflective imaging spectrometer
US7031059 *Sep 8, 2000Apr 18, 2006Centre National D'etudes SpatialesDevice for acquiring stereoscopic images
US7080912Jan 12, 2005Jul 25, 2006Raytheon CompanyHigh-resolution, all-reflective imaging spectrometer
US7209285 *Sep 11, 2003Apr 24, 2007Lockheed Martin CorporationCommon axis three mirror anastigmatic optic
US7382498Apr 30, 2007Jun 3, 2008Raytheon CompanyTwo-channel imaging spectrometer utilizing shared objective, collimating, and imaging optics
US7390101Jan 31, 2005Jun 24, 2008The Boeing CompanyOff-axis two-mirror re-imaging infrared telescope
US7703932Apr 27, 2007Apr 27, 2010Raytheon CompanyAll-reflective, wide-field-of-view, inverse-telephoto optical system with external posterior aperture stop and long back focal length
US7961386May 13, 2007Jun 14, 2011Rafael Advanced Defense Systems Ltd.Low orbit missile-shaped satellite for electro-optical earth surveillance and other missions
US8123371Feb 12, 2009Feb 28, 2012Raytheon CompanyAll-reflective afocal telescope derived from the first two mirrors of a focal three-mirror anastigmat telescope
US8277060Oct 2, 2012Raytheon CompanyApparatus and method of shaping a laser beam profile
US8411268Oct 1, 2010Apr 2, 2013Raytheon CompanyTwo material achromatic prism
US8422011Apr 16, 2013Raytheon CompanyTwo material achromatic prism
US8427744Apr 23, 2013Raytheon CompanyAll-reflective relayed focal telescope derived from the first two mirrors of an afocal three-mirror anastigmat
US8451318May 28, 2013Remotereality CorporationThree-mirror panoramic camera
US8534851Jul 20, 2010Sep 17, 2013Raytheon CompanyMultiple path substantially symmetric three-mirror anastigmat
US8953084 *May 30, 2012Feb 10, 2015Digimarc CorporationPlural focal-plane imaging
US9134518 *Sep 23, 2011Sep 15, 2015Lockheed Martin CorporationMultiple-sensor common-interface telescope
US20040021934 *Feb 25, 2003Feb 5, 2004Cook Lacy G.High-resolution, all-reflective imaging spectrometer
US20050134844 *Jan 12, 2005Jun 23, 2005Cook Lacy G.High-resolution, all-reflective imaging spectrometer
US20060171022 *Jan 31, 2005Aug 3, 2006The Boeing CompanyTwo mirror re-imaging telescope
US20070177261 *Dec 6, 2006Aug 2, 2007Murdock Steven GCatadioptric telescopes
US20080266687 *Apr 27, 2007Oct 30, 2008Raytheon CompanyAll-reflective, wide-field-of-view, inverse-telephoto optical system with external posterior aperture stop and long back focal length
US20090251773 *May 13, 2007Oct 8, 2009Yochay DanzigerLow orbit missile-shaped satellite for electro-optical earth surveillance and other missions
US20100188762 *Jan 26, 2009Jul 29, 2010Raytheon CompanyApparatus and method of shaping a laser beam profile
US20100201781 *Aug 13, 2009Aug 12, 2010Remotereality CorporationThree-mirror panoramic camera
US20100202073 *Aug 12, 2010Raytheon CompanyAll-reflective afocal telescope derived from the first two mirrors of a focal three-mirror anastigmat telescope
US20110085235 *Oct 12, 2009Apr 14, 2011Raytheon CompanyAll-reflective relayed focal telescope derived from the first two mirrors of an afocal three-mirror anastigmat
US20130321668 *May 30, 2012Dec 5, 2013Ajith KamathPlural Focal-Plane Imaging
CN101782680BJan 16, 2009May 23, 2012中国科学院西安光学精密机械研究所Optical system of total reflection
CN102955245A *Aug 22, 2011Mar 6, 2013朱沛伦Orthogonal telescope
DE4107576A1 *Mar 7, 1991Sep 10, 1992Frank GallertAnastigmatic three-mirror system for astronomic photography - has parabolic, spherical-domed and spherical-hollow mirrors
DE4226723A1 *Aug 10, 1992Feb 24, 1994Frank GallertAplanatic and anastigmatic mirror system with even picture field - uses collecting hyperbolic mirror, diffusing parabolic mirror, with light bunch falling finally on collecting spherical mirror.
DE4229874A1 *Sep 4, 1992Mar 10, 1994Gallert FrankAplanatic, anastigmatic, distortion-free mirror system with even picture field - comprises three mirror system with elliptical main mirror and hyperbolic diffusion mirror arranged in cassegrain configuration producing intermediate picture
EP0019447A1 *May 13, 1980Nov 26, 1980Hughes Aircraft CompanyThree mirror anastigmatic optical system
EP0458130A2 *May 7, 1991Nov 27, 1991Hughes Aircraft CompanyWide field all-reflective multiple field of view telescope
EP0512439A1 *May 1, 1992Nov 11, 1992Hughes Aircraft CompanyAll-reflective optical system directing light to a plurality of viewing regions
EP0676656A1 *May 1, 1992Oct 11, 1995Hughes Aircraft CompanyReflective optical system having multiple simultaneous viewing functions
EP1772761A1Oct 5, 2005Apr 11, 2007EADS Astrium GmbHUltra-wide field multiple-mirror apparatus
EP2016755A2 *May 13, 2007Jan 21, 2009Rafael-Armament Development Authority Ltd.Low orbit missile-shaped satellite for electro-optical earth survellance and other missions
EP2169441A1Sep 22, 2009Mar 31, 2010Astrium SASKorsch-type telescope with bounce mirrors
EP2312366A1Aug 11, 2010Apr 20, 2011Raytheon CompanyAll-reflective relayed focal telescope derived from the first two mirrors of an afocal three-mirror anastigmat.
WO1995010793A1 *Oct 17, 1994Apr 20, 1995Matra Marconi Space France S.A.Telescope for infrared or visible imaging
WO1995034013A1 *Jun 7, 1995Dec 14, 1995Industrial Research LimitedHigh speed optical system
WO2010093631A1Feb 9, 2010Aug 19, 2010Raytheon CompanyAal-reflective afocal four-mirror telescope
Classifications
U.S. Classification359/366, 359/859
International ClassificationG02B17/06, G02B23/06
Cooperative ClassificationG02B17/0631, G02B23/06
European ClassificationG02B17/06B1, G02B23/06